Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is disassociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perflurosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The combination of the anode, cathode and membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Many fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas as a flow of air, typically forced through the stack by a compressor. Not all of the oxygen in the air is consumed by the stack, and some of the air is output as a cathode exhaust gas that may include water as a stack by-product.
The fuel cell stack needs to put out a certain amount of power to provide the desired work. The fuel cell stack power is determined by the amount of oxygen applied to the cathode relative to the amount of hydrogen applied to the anode. The amount of oxygen required to achieve a certain stack output power is referred to as cathode air stoichiometry or cathode lambda. Particularly, the cathode lambda is the amount of oxygen delivered to the stack, divided by the amount of oxygen that is consumed by the stack. Some fuel cell systems operate at a constant cathode lambda across the entire power output of the system. Other fuel cell systems operate at different cathode lambdas for different power outputs.
Some fuel cell systems incorporate a humidifier to humidify the air caused to flow to the cathode. If the humidifier contains a leak, the leak causes a loss of control of the stoichiometry in the air flowing to the cathode. It is undesirable for the fuel cell stack to be operated with an air flow having an incorrect cathode stoichiometry.
It is known in the art to employ an air flow meter that measures the air flow applied to the compressor to determine the amount of oxygen that is being applied to the stack. It is also known to employ an amp meter to measure the current output of the stack. The combination of the oxygen applied to the stack and the current output of the stack can be used to determine the cathode lambda at which the system is operating. A controller operates the compressor at the desired speed to achieve the proper cathode lambda.
It would be desirable to provide a fuel cell system including an oxygen sensor and a humidifier, wherein the fuel cell system is adapted to provide a sensor signal indicative of an oxygen concentration in the air flow exiting the fuel cell stack to determine if a leak exists in the fuel cell system.